Opioid addiction is a national epidemic contributing to the deadliest drug overdose crisis in US history and accompanied by excessive healthcare burdens due to the misuse of heroin and opioid prescription medications. There continues to be a lack of neurobiological knowledge about opioid use disorder to drive novel therapies critically needed to provide options to the current medications that are predominantly opioid- based and thus of abuse liability themselves. A fundamental core of our reverse translational research efforts has been to fill critical gaps of knowledge by direct investigation of the brains of human heroin abusers. Through such strategies we recently discovered a previously unrecognized neurobiological impact of opioids suggesting opioid-induced epigenetic and neurodegenerative pathology linked to the FYN gene and hyper- phosphorylated tau (pTau); we also detected pTau-neurofibrillary pathology in heroin abusers. FYN is a Src family tyrosine kinase within the postsynaptic density that phosphorylates tau, involved in microtubule stability and dynamics, and thus a regulator of cytoskeletal remodeling which is a key feature of addiction. We verified elevation of FYN-targeted Tau phosphorylation in rats that self-administered heroin and in our chronic opioid in vitro cell culture model. Moreover, were able to inhibit heroin self-administration behavior in animals treated with a FYN inhibitor. These fascinating multidisciplinary and integrative data provides a strong foundation on which to interrogate FYN and pTau pathology in opioid abuse with the goal of therapeutic development. We hypothesize that upregulation of FYN and the resulting pathological pTau impairments in mesocorticolimbic brain areas contribute to heroin addiction behavior and can be targeted for treatment interventions. We propose to: (1) determine the molecular signature of FYN and related pTau networks in mesocorticolimbic regions associated with heroin abuse by RNA-sequencing in a cell-specific manner, (2) characterize the mechanisms by which opioids lead to FYN disturbances and hyper-pTau neuropathological signatures. Such mechanisms focus on epigenetic alterations of FYN promoter, characterizing intracellular signaling cascades linked to FYN and Tau, and monitoring in vivo Ca2+ fluctuations (measured by fiber photometry) linked to excitotoxicity, and (3) develop FYN-related treatments to reduce heroin self-administration behavior. Results gained from our integrative multidisciplinary study will advance knowledge of FYN-related abnormalities underlying opioid abuse and provide new science-based pharmacotherapeutic tools to expand treatment options for opioid addiction.